The present invention relates to a process for passivation of photoconductive detectors, made in particular of Hg Cd Te, used for the detection of infra-red radiations in a spectral domain depending on the proportion of Cd with respect to the Hg Cd assembly.
For a proportion close to 30%, the detection band extends from 3 to 5 .mu.m, and for a proportion close to 20%, the band extends from 8 to 12 .mu.m.
In order to avoid pollution thereof, these detectors are subjected to a surface passivation.
A process of passivation by deposit on the surface of the conductors of a layer of a chemical compound, such as for example zinc sulfide or silicon oxide, is known. Unfortunately, with such a process, the detectors are not very reliable, their performances, essentially their sensitivity and detectivity, degrading in time, particularly at a high temperature, greater in the present case than 70.degree. C.
Now, both the sensitivity and detectivity of a photoconductive detector depend on the effective lifetime of the carriers which, itself, is inversely proportional to the surface recombination velocity of the carriers provoked by the incident radiation.
As the length of diffusion of the holes is notably greater than the thickness of the detector between its two opposite faces, recombination of the carriers is provoked by these two faces.
As this surface recombination velocity, which must therefore be as low as possible, depends on the surface potential, the latter must therefore be maintained stable for the detector to be reliable.
U.S. Pat. No. 3,977,018 proposes a process for minimizing and stabilizing the surface recombination velocity of the two sides of a photoconductive detector, with a surface charge density greater than that in volume. This process consists, on a material, for example of n type, in forming on the surface a passivation layer with positive charge density, in order thus to create on the surface a layer of type n+. The dipoles contained in this layer are oriented with their positive charges disposed on the inside of the material, and repel the minority carriers of the material constituted by the holes. The recombination of the latter on the surface is therefore reduced, whilst their effective lifetime is increased.
More precisely, this Patent proposes to make a previously formed layer of native oxide grow on at least one of the faces of the detector, by electrochemical method and, more particularly, by anodic oxidation of the material in an electrolyte.
Although the process of this Patent makes it possible to produce passivated infrared photoconductive detectors with long and stable lifetime at high temperatures, it nonetheless presents several drawbacks.
Firstly it is relatively complex to carry out in view of its electrochemical nature. Secondly, a particular embodiment of the process must correspond to each type of detector material. Finally, certain performances of the detectors obtained by this process are not yet totally satisfactory, particularly their detectivity.